Semiconductor devices such as field effect transistors are employed as switching devices for radio frequency (RF) signals in analog and RF applications. Semiconductor-on-insulator (SOI) substrates are typically employed for such applications since parasitic coupling between devices through the substrate is reduced due to the low dielectric-constant of a buried insulator layer. By providing the buried insulator layer of SOI technologies, which has a dielectric-constant less than the dielectric constant of a semiconductor material in a bulk substrate, the SOI substrate reduces capacitive coupling between an individual semiconductor device and the substrate, and consequently, reduces secondary capacitive losses to the substrate.
However, even with the use of a SOI substrate, the secondary capacitive coupling of electrical signals between semiconductor devices is significant due to the high frequency range employed in the radio frequency applications, which may be, for example, from about 900 MHz to about 80 GHz, and may include even higher frequency ranges. This is because the effect of capacitive coupling between electrical components increases linearly with frequency.
For a RF switch formed on an SOI substrate, the semiconductor devices comprising the RF switch and the signal processing units in a top semiconductor layer are capacitively coupled through the buried insulator layer to a bottom semiconductor layer. Even if the semiconductor devices in the top semiconductor layer employ a power supply voltage from about 3V to about 9V, the transient signals and signal reflections in an antenna circuitry may increase the actual voltage in the top semiconductor layer up to as high as 30V. Such voltage conditions induce a significant capacitive signal-voltage coupling between the semiconductor devices subjected to such high voltage signals and an induced charge layer within an upper portion of the bottom semiconductor layer, which changes in charge density and charge polarity at the frequency of the RF signal in the semiconductor devices in the top semiconductor layer.
The induced charge layer capacitively couples with other semiconductor devices in the top semiconductor layer including the semiconductor devices that an RF switch is supposed to isolate electrically. The spurious capacitive coupling between the induced charge layer in the bottom semiconductor layer and the other semiconductor devices provides a secondary capacitive coupling, which is a parasitic coupling that reduces the effectiveness of the RF switch. In this case, the RF signal is applied to the other semiconductor devices through the secondary capacitive coupling although the RF switch is turned off.
Further, during one half of each frequency cycle of the RF signal, the top portion of the bottom semiconductor layer directly underneath the buried insulator layer is in a depletion condition, in which charge carriers in the bottom semiconductor layer are repelled from the bottom surface of the buried insulator layer. For example, when the conductivity type of the bottom semiconductor layer is p-type and the voltage of the top semiconductor portions is positive relative to the voltage at the bottom semiconductor layer, or when the conductivity type of the bottom semiconductor layer is n-type and the voltage of the top semiconductor portions is negative relative to the voltage at the bottom semiconductor layer, the majority charge carriers, i.e., holes if the bottom semiconductor layer is p-type or electrons if the bottom semiconductor layer is n-type, are repelled from the upper portion of the bottom semiconductor layer to form the induced charge layer, which is depleted of the majority charges.
Moreover, when the magnitude of the voltage differential between the top semiconductor portions and the bottom semiconductor layer is sufficiently great, an inversion layer including minority charges, i.e., electrons if the bottom semiconductor layer is p-type or holes if the bottom semiconductor layer is n-type, is formed within the induced charge layer. The inversion of the semiconductor portions adds to RF coupling to source/drain regions, degrades isolation, increases harmonic distortion and produces erratic DC behavior (e.g. kinks, non-monotonic Vt (Vbg), layout sensitivities).